A Study of the Physical Mechanisms Responsible for the Nonlinearity of the Flow Characteristics of Low-Pressure Gas-Phase Injectors

Environmental regulations and stricter emission limits are driving the development of advanced fuel supply systems. Precise fuel metering under varying engine loads has become critical, with modern strategies using multiple injections of very short duration. However, injector behavior, particularly nonlinear flow characteristics, is not fully understood. This study presents an experimental analysis of the flow characteristics Q = f (tinj) and opening dynamics of five low-pressure gas injectors with different valve system designs. The tests were conducted for injection times tinj = 0–20 ms. For tinj > 2.5 ms, the characteristics were very well described by a linear model (R2 > 0.995), whereas for tinj < 2.5 ms, there was a clear deviation from the linear relationship between flow rate and injection time. Analysis of the electrical signals, outlet pressure, and body vibrations made it possible to identify the mechanistic sources of the observed nonlinearity. It was demonstrated that the initial lack of flow results from an electromechanical delay associated with the rise in current and the electromagnetic force required to overcome the spring force, friction, and inertia of the valve element. The subsequent movement of the valve contributing factors a dynamic change in the flow cross-sectional area and, consequently, a nonlinear change in flow rate. Additionally, the change in the position of the valve element affects the inductance of the coil and the nature of the electromagnetic force. Near the maximum lift, the element bounces off the stop, causing a momentary change in its position and a local decrease in flow rate. Only after the valve element’s motion stabilizes does the flow transition to a nearly linear relationship. The response times of the injectors ranged from 0.60 to 1.30 ms, and the times to reach full opening ranged from 1.08 to 2.14 ms, corresponding, respectively, to the onset and the transition to the steady-state region of the characteristic curve. The results indicate that the nonlinearity of the short-time portion of the characteristic has a mechanistic, electromechanical nature and results from the coupling of electromagnetic phenomena, the motion of the valve element, and the varying flow cross-section. This means that accurately modeling it requires taking into account the actual dynamics of valve-opening, particularly in the case of strategies that use short and repeated injection pulses. These findings highlight a significant limitation in fuel dosing precision and emphasize the need to incorporate nonlinear injector models or dynamic corrections in ECU control algorithms—an essential step for further reducing exhaust emissions.